AU Bio/Headcanons

I’m still working on this stuff—but I wanted to make mention of the main foundation(s) for my AU version of Ten, whenever I'm writing him in that form. There are two critical events that are the most important and set the stage for how everything diverges.

1.  THE END OF TIME
The Doctor does not regenerate in this scenario. He still makes his final tour and visits everyone, including past Rose. But when he starts to succumb to the radiation and falls in the snow, he sees Rose (in the form of the Bad Wolf) instead of the Ood. The “real” Rose is still in the parallel world. When she absorbed the time vortex and the Bad Wolf scattered her essence across time and space, she foresaw the necessity of the Tenth Doctor's continued presence in the universe. For some reason, events weren’t happening as they should be, and it was not supposed to be his time. 

This version is a manifestation of Rose, imbued with remnants of the Bad Wolf’s abilities. Essentially, she is an echo. An aspect of herself that was left behind to intervene at this critical juncture, appearing to him in his moment of need. She helps him back to the TARDIS, then heals him through a combination of her touch and a kiss; reminiscent of the way the Ninth Doctor had saved Rose on Satellite Five. It comes back full-circle. She essentially resets his current incarnation back to prime condition, without triggering a regeneration. She vanishes shortly after.

2.  TRENZALORE 
Considering his escape from regeneration, it’s the Tenth Doctor who faces the centuries-long siege to protect the town of Christmas. The same events still happen; he spends about 900 years there and grows old. But when Clara Oswald pleads to the Time Lords for assistance, she requests not only a new set of regenerations for the Doctor, but also that he retains his current form (knowing that he is fond of his current incarnation). Her plea is granted, "resetting the clock" on his age and allowing him a renewed continuation of his current form.

By the time the Doctor crosses paths with Donna Noble again, only about 60-100 years have passed from his perspective, between Trenzalore and their reunion (excluding his time spent in the confession dial). As a result, he appears mostly the same as he had before, with minimal signs of aging. Of course, on a psychological level, considering canon events from this point in time through to the 60th Anniversary specials, he has still been through a *lot*. So he obviously goes through some changes in that regard, to an extent, based on his experiences. 

So, there you have it. It also explains why Ten still looks like—well, Ten… and why he has the same underlying personality, despite the fact that well over 1,000 years have passed for him in the span of the 15 years that have passed for Donna Noble. I realized that it wouldn’t have made sense without this extra tweak.

I mean… alternatively, sure. I could've just assumed that everything happened as canon, the only difference being that Thirteen regenerated back into Ten.  But I find Option #1 to be more fun. Though they share the “same face”, Ten and Fourteen are not interchangeable. They still have some distinct differences and are not quite the same person. I don’t write as the Fourteenth Doctor. I have nothing against him, I’m just more comfortable with Ten. I also still write as canon Ten quite often, so I need to leave the door open for that. This whole ‘AU’ thing just gives me a way to involve him in more recent events in the timeline of the show when the situation calls for it. And it allows me to keep the muse relevant without always having to rely on time displacement.. With that SAID: I'm still open to writing Ten in current events as canon sometimes, too. If it works for the story. 

If the story isn't based on recent events, I can write him anywhere. Canon, non-canon, prime universe, Pete's World, whatever. I'm pretty flexible. 

Side note: I do not like the Timeless Child arc, and I prefer not to write about it. Sorry. I just don't like the idea, and I prefer to keep his Gallifreyan heritage intact in my own writing. Might be a controversial take, I don't know. But if you don't like it, no one's forcing you to write with me. I might not be the most accurate writer in the world with the lore, but hey. It's fiction. We're all here to have fun. So, let's have fun!.